uow: possible solution for the rollback problem

* the problem is that the Session.rollback does rollback the parent too
  which is not the prefered situation here.

* it could be that this is only necessary for the tests and not for
  production but that has to be tested

* see https://github.com/sqlalchemy/sqlalchemy/blob/rel_2_0_36/lib/sqlalchemy/orm/session.py#L1971

* it seems that it is not configurable so the private method has to be
  used

invenio_db/uow.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 #
 # Copyright (C) 2024 CERN.
+# Copyright (C) 2024 Graz University of Technology.
 #
 # Invenio is free software; you can redistribute it and/or modify it
 # under the terms of the MIT License; see LICENSE file for more details.
@@ -167,6 +168,7 @@ def __init__(self, session=None):
 
     def __enter__(self):
         """Entering the context."""
+        self.session.begin_nested()
         return self
 
     def __exit__(self, exc_type, exc_value, traceback):
@@ -199,7 +201,7 @@ def commit(self):
 
     def rollback(self, exception=None):
         """Rollback the database session."""
-        self.session.rollback()
+        self.session._transaction.rollback(_to_root=False)
 
         # Run exception operations
         if exception: